Microstructural Analysis of Co-Free Maraging Steel .Maraging steels are being investigated because

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  • J. Aerosp. Technol. Manag., So Jos dos Campos, Vol.6, No 4, pp.389-394, Oct.-Dec., 2014

    ABSTRACT: Maraging steels have low carbon content and are highly alloyed, having as main feature the ability to increase the mechanical strength after thermal aging. Therefore, the objective of this work is to analyze the effect of aging in a Co-free maraging steel, Fe 0.014% C 0.3% Mn 3.9% Mo 2.1% Cu 0.19% Si 11.8% Cr 9.1% Ni 1.0% Ti (wt), at different heat treatment times (10 min - 960 min) at constant temperature (550C), after cold rolling up to 66% and 77% area reduction. Microstructures were analyzed by scanning electron microscopy (SEM - EDS) and mechanical properties by Vickers hardness measurements. The results showed a significant increase in the hardness of the material after aging heat treatment on the solution treated and deformed samples. The aging heat treatment which was harder about 650 HV was 550C/60 min.

    KEYWORDS: Maraging steel, Cold rolling, Hardness test Aging.

    Microstructural Analysis of Co-Free Maraging Steel AgedAline Castilho Rodrigues1, Heide Heloise Bernardi1, Jorge Otubo2

    INTRODUCTION

    Maraging steel was developed in the 1950s, to support the demand for a low density and temperature resistant steel (250C300C), for improving aircraft performance. Thisnew material was created adding aluminum (Al) and titanium (Ti) in steel with nickel (Ni). In the 1960s, cobalt (Co) and molybdenum (Mo) were added into the composition of maraging steels. Clarence George Bieber, in International Nickey Company, studied the addition of these elements in the alloy and concluded that there is a significant increasing in mechanical resistance (Lopes, 2007; Mndez, 2000). The classification of commercial maraging steels, according to the percentage of nickel, is: 18Ni (250), 18Ni (300) and 18Ni (350) (Lopes, 2007).

    Maraging steel received this name due to two words: martensite and aging, which mean martensite aged. Low carbon and alloyed, the maraging steels are capable of increasing their mechanical resistance and hardness after aging heat treatments. The treated solution state has a martensitic structure with high ductility and toughness, and it can be modified after aging heat treatments (Cardoso etal., 2013a). Due to the ductility of martensite, the maraging steel allows cold mechanical forming, not requiring intermediate heat treatment. A mechanical processing technique that can be used in maraging steel is cold rolling.

    During the cold rolling, plastic deformation and increase of hardness occur, therefore, there is an increasing of dislocation density and crystalline defects, hindering the movement between them. Due to the plastic deformation, there is an increase in tensile strength and yield point (Callister, 2012). On the other hand, these mechanical properties can also be improved by thermal aging, in which the formation of intermetallic precipitates occurs. In Table 1,the types of precipitates which may be formed

    doi: 10.5028/jatm.v6i4.400

    1.Faculdade de Tecnologia de So Jos dos Campos So Jos dos Campos/SP Brazil 2.Instituto Tecnolgico de Aeronutica So Jos dos Campos/SP Brazil

    Author for correspondence: Aline Castilho Rodrigues Faculdade de Tecnologia de So Jos dos Campos FATEC Prof. Jessen Vidal | Av. Cesare Mansueto Giulio Lattes, s/n, So Jos dos Campos/SP | CEP: 12.247-014 Brazil | Email: alinerodrigues_1@msn.com

    Received: 08/02/2014 | Accepted: 09/30/2014

  • J. Aerosp. Technol. Manag., So Jos dos Campos, Vol.6, No 4, pp.389-394, Oct.-Dec., 2014

    390Rodrigues, A.C., Bernardi, H.H. and Otubo, J.

    on maraging steels, depending on the alloy composition, are shown (Padial etal., 2000).

    Steel with martensitic structure, when subjected to a heat treatment at a constant temperature, suffers a growth of intermetallic precipitates uniformly in the matrix (Carvalho etal., 2013). Theprecipitate size is a contributing factor to the increased mechanical resistance of maraging steels. The smaller the precipitates and the more coherent with the matrix, the greater the hardness of the alloy, since there is less space between the particles; furthermore, they act as barriers to movement of dislocations (Padial etal., 2000).

    The alloying elements of maraging steel, such as molybdenum, titanium, nickel, cobalt, aluminum, and others, have a great influence on the mechanical properties of this alloy, in other words, they are responsible by precipitates hardeners (Carvalhoetal., 2013). However, recent studies (Hu etal., 2008; Leitner etal., 2011; Mahmoudi etal., 2011; Mahmudi etal.,2011; Nili-Ahmadabadi, 2008; Schnitzer etal., 2010; Shaetal., 2013) are being conducted in maraging steels without cobalt addition, in order to reduce the production costs of this alloy. Maraging steels are considered expensive due to their materials preparation and expensive alloy elements such as nickel and cobalt processes. Theelimination of the cobalt content and the substitution of nickel by cheaper elements, such as manganese, has been studied. Cobalt is used only to minimize the solubility of molybdenum. In the case of maraging steel studied, this element was replaced by chrome, which besides

    improving the hardenability increases the corrosion resistance (Nili-Ahmadabadi, 2008).

    Maraging steels are mainly applied in the aeronautical, aerospace, nuclear and military industries, as they have great advantages such as good weldability, high strength, high yield strength, high fracture toughness, they support high working temperatures, have good machinability, good formability, among others, with great applicability matrices and tools (Lopes, 2007).

    High-tensile steels such as 300M and SAE 4340 have extensiveapplication in aeronautics and aerospace industry, being mainly applied in the manufacture of structural components (Zang etal., 2013; Boakye-Yiadom etal., 2014).

    Maraging steels are being investigated because of the possibility of replacing the commercial alloy steels, 300M and 4340 for example, widely used in aeronautical industry (Carvalho etal., 2013). Thus, this study aims to examine the effect of aging heat treatment in non-commercial maraging steel (Co-free), in order to compare it to the 300M and 4340 steels.

    EXPERIMENTAL

    The maraging steel used in this work was produced by vacuum induction melting. The ingots was hot forged and then hot rolled until we obtained bars of 20x20mm of dimension and chemical composition: Fe 0.014% C 0.3% Mn 3.9% Mo 2.1% Cu 0.19% Si 11.8% Cr 9.1% Ni 1.0% Ti (wt.%).

    Its bar (20x20 mm) was separated into two parts resulting in two conditions: Condition I (C-I): bar dimension of 20 x 20 x 10 mm; Condition II (C-II): bars of 20 x 20 x 10 mm (~100 wt.),

    remelted (1 remelt) on an arc furnace, resulting in an ingot dimension of 16x10x95 mm.

    Both conditions (C-I and C-II) were solution treated at 1050C during 1 hour, quenched in water at room temperature and then cold rolled to a plate of 3 mm in thickness, resulting in 66 and 77% of area reduction (AR) for C-I and C-II, respectively. The deformed samples (AR = 66 and 77%) were cut in several parts and each part was annealed at 550C for different times, ranging from 10 to 960 min.

    Microstructural characterization was performed using a Scanning Electron Microscope (SEM) VEGA 3 TESCAN model, equipped with an energy dispersive spectrometer system (EDS). Vickers hardness

    Table 1. Precipitates hardeners formed during the aging of maraging steels (Padial etal., 2000).

    Phase Crystalline Structure

    Ni3Mo Orthorhombic

    Ni3Ti Hexagonal ordered

    Ni3V Hexagonal compact

    Ni3W Orthorhombic

    Fe2(Mo, Ti) Type hexagonal laves

    FeMo Tetragonal

    FeTi CsCl type cubic

    Fe7Mo4 Hexagonal

    R (Mo-Co-Cr) Hexagonal rhombic

    X (Fe-Cr-Mo) Body-centered cubic

  • J. Aerosp. Technol. Manag., So Jos dos Campos, Vol.6, No 4, pp.389-394, Oct.-Dec., 2014

    391Microstructural Analysis of Co-Free Maraging Steel Aged

    testing was performed using a Future-Tech FM-7000 microindenter with a load of 200g/15s, in which the results correspond to the mean of ten values taken on each specimen. Themicrograph analyses and the Vickers hardness were undertaken in a longitudinal section of the samples (wire rolling direction).

    RESULTS AND DISCUSSION

    Figure 1 shows the hardness variation as a function of heat treatment time for maraging steel with ARs = 66 and 77%. For solution treated (C-I and C-II) and deformed samples, the hardness values are represented at zero time in Fig. 1.

    Solution treated samples (C-I and C-II) showed average hardness values of 250 and 300HV, respectively. Comparing the solution treated to the deformed condition, it is observed an increase around 50% in average hardness values (HV66%=425

    and HV77% = 407), corresponding to the effect of work hardening. About thermal aging, there is a significant increase in hardness at relatively low heat treating times (Fig. 1). For both area reductions, increased hardness around 200HV occurs with only 10 min thermal aging. Heetal. (2002) observed that the heat treatment at 470C performed in a maraging steel with 18%wt. Ni, after forging, has achieved a 90% increase in hardness, with only 15 min of aging.

    In the detail of Fig. 1, it is observed that the maximum hardness is obtained at 550C/60 min for both ARs. On the other hand, the thermal aging at 550C exhibits a decrease in hardness values after 240 min for deformed sample with AR=66%, whereas a decrease it is observed after 90 min to condition AR = 77%. Therefore, it can be said that in smaller ARs, there is an increase in aging time of the alloy, delaying the onset of overaging. Based on the average hardness values, the processing) in the matrix, resulting in a lath martensitic structure (Mahmoudi etal.